In small rocket propelled missile systems such as used in point defense weapons, it is desirable to have two or more separately ignited thrust pulses supplied by the rocket motor in order to achieve selectable kinetic management capability and the required maneuverability for the missile at high altitude and in long range air to air engagement. To provide these capabilities, two or more radially or end burning solid propellant segments or grains, or combinations thereof, are ignited and burned in sequence. A tandem arrangement of the propellant grains within the rocket motor casing is the most common configuration, although a radial arrangement of successively burned grains could also be employed. A critical aspect of such a multiple grain design is provision for the thermal protection of individual propellant grains during the burning of the preceding grain or grains, and during any inter-pulse interval, in order to prevent premature ignition of the propellant grain which could result in destroying the propulsion characteristics of the rocket motor or the motor itself.
Currently available designs of thermal protection for solid propellant rocket grains are based upon enclosing the propellant burning surface in a thermal layer constructed of an elastomer such as butyl, polyisoprene, or polybutadiene with insulating material dispersed therein such as silica and asbestos. While such materials provide requisite thermal protection, they present a nearly unsolvable problem following completion of their design function in small rocket motors since they must either be successfully retained in the rocket motor casing or expelled through the rocket motor nozzle throat. The first alternative is rarely accomplished with consistent success, and the second alternative is successful only when the nozzle throat is large enough to permit ejection of the protective thermal material. With small nozzle throats under one inch in diameter such as are encountered in anti-missile point defense systems, a throat blockage may lead to a potentially catastrophic motor chamber pressure spike or to combustion instability. With such small nozzles the strength of the elastomer used in the required thickness to provide thermal protection prevents the layer breaking up into small enough pieces to be ejected through the motor nozzle.
To provide the necessary inter-grain thermal protection while overcoming the foregoing disadvantages, it is desirable to provide a thermal barrier for individual propellant grain combustion surfaces, either alone or in combination with an elastomer layer which has mechanical properties allowing fracture of the barrier on ignition of the protected propellant grain into fragments sufficiently small to pass through nozzle throats of less than one inch in diameter. Applicant's invention meets this criteria in addition to providing other valuable characteristics that minimize interference with the motor design and propellant burning characteristics.